Previous studies of mating interactions between schistosome species from different evolutionary lineages in mice showed that mating is not random: heterospecific pairs form but, given the choice, the preference is for homospecific partners. Mating competition and change of mate were also demonstrated. Schistosoma mansoni is a medically important parasite of humans belonging to a different evolutionary group from S. margrebowiei, a parasite of bovines of veterinary importance. Experiments were designed to investigate the mating behaviour of these two species in mice. Analysis of the data showed a preponderance of intraspecific pairs over interspecific, demonstrating a specific mate preference system for both species. The existence of mating competition between these species was indicated. Males of both species were able to actively compete for paired females by pulling them away from their partners, thus effecting a change of mate. Overall, neither species appeared to be competitively dominant to the other, and the data suggest that where this is the case in sequential infections, the most successful species in terms of worm return and ability to form pairs will be the first species to infect the host.